1. Field of the Invention
This invention relates to pipeline inspection vehicles of the type which can be inserted into a pipeline of ferro-magnetic material and carried along by the fluid flow in the pipeline to detect defects in the pipeline wall, and in particular the invention relates to the magnet assemblies used in such vehicles.
2. The Prior Art
Pipeline inspection vehicles of the kind with which the present invention is particularly concerned usually include a plurality of magnet assemblies supported around the body of the vehicle, which assemblies are arranged to engage the internal surface of the pipeline wall during movement of the vehicle along the pipeline under test so as to permeate an area of the wall with a magnetic flux and thereafter detect any anomalies in the magnetic field adjacent the wall caused by flux leakage from defects in the wall. Usually, each magnet assembly consists of a magnet and a coupling member of flux-conducting material suitably shaped for coupling the magnet to the wall so that a magnetic circuit is obtained.
The magnetic requirements for a coupling member are that the magnetic reluctance thereof should be low, i.e., to minimize the strength requirement of the magnet, and it should not vary significantly when traversing obstructions, for example, welds or steps in the pipeline bore or when passing around bends in the pipeline itself.
In practice, these requirements have been adequately fulfilled by using coupling members composed of steel bristles. The bristles are densely packed to give a large metal area, and the length and diameter of the bristle wires are optimized to give an adequate combination of stiffness and flexibility. The stiffness is required to provide load-carrying capacity, whereas the flexibility is required to provide a smooth transition over girth welds and other obstructions in the pipeline wall.
There are a number of disadvantages associated with such steel bristle members. For example, the manufacturing costs associated with producing the very densely packed bristles are high and the service life is limited due to abrasive wear in use. This means that the bristle replacement costs are significant. Breakage of individual wires, caused by weakening due to corrosion pitting (caused by water in the pipeline or associated with the steam cleaning process to which the vehicles are subjected after a service run in the pipeline), is also a problem.
It is an object of the present invention to provide an improved magnetic coupling member of a magnet assembly for a pipeline inspection vehicle which, as compared to steel bristle coupling members, is cheaper to construct; is easier to disassemble from the magnet assembly and replace when required; has more metal per unit area and hence lower reluctance; enables a wider choice of metal or metal alloys to be used; and is less subject to corrosion and more resistant to abrasive wear.